As is known, Raschel-type linear knitting machines are provided with a plurality of bars designed to carry a plurality of thread-holding elements, commonly known as thread-guides. Said bars should be moved so as to enable the threads associated to the thread-guides to be correctly fed onto the needles of the knitting machine for the formation of new fabric with the well-known technique in which the new thread enters the old loop and the old loop is discharged and becomes part of the fabric being formed. In order to achieve its knitting task, the thread-guide bar makes two basic movements simultaneously, i.e. a first linear movement in front of the hook of each needle, commonly known as “shog”, and an oscillating movement on the side of each needle for bringing the threads alternatively before and behind the needle hook, commonly known as “swing”.
The present invention relates to a device for enabling the oscillating movement (“swing”) for the thread-guides.
Currently, in linear knitting machines the oscillation of the thread-guide bars, which is usually of 4° to 10°, is obtained by means of several methods, all of which exploit leverage systems, such as quadrilaterals, suitably connected to one another and derived from systems for handling the rising and descent of needles for the formation of the knitted stitch, as is shown for instance in documents WO 03/071018 and U.S. Pat. No. 3,221,520. Accordingly, the whole mechanism of the machine is rigorously synchronized in its basic movements, whatever the speed at which the machine is running.
As is known, thread-guide bars, eight of them being generally present on double needle-bed machines, are associated to at least one support, which is in its turn connected to said leverage systems for transmitting the oscillating movement thereof. Said bars are connected to two supports, each of them being placed on one of the end portions thereof. If necessary, it can further be provided for intermediate resting supports, which can both actively transmit the oscillating movement and be passively subjected to it.
As was already said, the leverages convert the linear movement resulting from the needles into an oscillating movement for the thread-guide bars. As a matter of fact, the oscillating movement is generated by the movement of a rod connected to the support of the thread-guide bars so as to make it rotate around the axis of the shaft supporting it. As a rule, as can be seen in FIG. 1, the support of the thread-guide bar is made up of a main body to which the bars as connected, and of a supporting arm, upon which the rod acts and which has a main axis basically perpendicular to the main axis of the main body. Moreover, the support is associated to the shaft supporting it on the point of connection between the arm and the main body, which is also the center of rotation for said support. This particular structure allows to obtain an oscillating movement for the main body starting from the linear movement of the arm obtained by means of the rod.
Known devices as disclosed above show various drawbacks. Firstly, the systems for transmitting motion from the motor of the machine to the thread-guide bars are quite complex, since they have to be extremely accurate because of the narrow spaces in which needles and thread-guides work with respect to the overall size of the machines, and require a very large number of components. This increases costs hugely. Moreover, the mechanical complexity of the devices strongly limits their speeds of use, and thus said machines often represent a bottleneck in the manufacturing system into which they are integrated.
Secondly, said devices have a very low flexibility, since it is very difficult to make after-changes to them because of their complexity. Even maintenance operation for repairing or replacing elements can be complex. Anyhow, these operations require the intervention of specialized personnel working for the company that has made the machines, with subsequent problems of production stops and further cost increase.
Eventually, another problem with known systems consists in the need to continuously invert the direction of movement of the support, and thus of the thread-guide bars, so as to make oscillations. As a matter of fact, the masses involved, which are quite high, have to be pushed in one direction, so as to create a counterclockwise oscillation for instance, then at stroke end they have to be braked and pushed in the opposite direction, so as to make the following clockwise oscillation for instance. Such a device, therefore, gives rise to several mechanical problems leading inevitably to solutions involving large overall sizes of stressed components and strong reductions of operating speeds. Moreover, said devices generate very strong vibrations that have to be absorbed by the machine through suitable measures, such as for instance big anti-vibration supporting structures.
The state of the art shows devices mitigating the problem disclosed above, though further increasing costs. They are basically made up of eccentric systems based on the principle of connecting rod-crank imparting a sinusoidal movement to the support, as shown in FIG. 2. The sinusoidal movement of the connecting rod slows down the stroke of the support on the point of inversion of the movement, thus greatly reducing vibrations and discharging the forces of inertia generated on the various mechanical connections as far as the motor.
Moreover, known knitting machines can include even more than two of the conventional devices associated to the ends of the thread-guide bars. For instance, in a machine with a needle-bed having a length of about 3.5 m, there can be 8 devices spaced from one another of about 0.5 m. As a matter of fact, the use of several devices enables to reduce size and, therefore, to obtain higher speeds of use. However, in this case the size of the motor and of the shaft connected thereto significantly increases, since eight of these devices are fitted onto the shaft, together with other devices involved in the movement of needles and other elements, which devices increase the forces of inertia involved due to the masses in movement that have to be moved in a suitable manner both at constant speed and during acceleration or braking.
It should be pointed out that, generally, these devices are located in the portion containing the rear needle-bed, thus leaving the front portion of the machine free for different reasons, also of economical nature. Therefore, the system is not balanced and gives rise to vibrations occurring also at low speeds (350 oscillations per minute for instance).